1. Field of the Invention
The present invention relates to a resin-coated carrier for an electrophotographic developer which constitutes a two-component developer used for development of an electrostatic latent image such as electrophotography, electrostatic recording or electrostatic printing, and to a two-component developer and a developer for replenishment using the same.
2. Related Background Art
Polymers having a hydrophilic group such as a sulfonic acid group are expected to be used for various applications.
The polymers containing a sulfonic acid group are generally synthesized only by using a specific vinyl monomer containing a sulfonic acid functional group. Specific examples of the monomer include sulfonated styrene and AMPS (2-acrylamido-2-methylpropanesulfonic acid). Such monomers are described in Japanese Patent Application Laid-Open No. 2002-351147.
Electrostatic printing comprising forming an electric latent image on the surface of a photoconductive material by electrostatic means and developing the latent image to form an image has been conventionally known, and various methods thereof are known. Specifically, electrostatic printing generally involves forming an electric latent image on a photosensitive member by various means using a photoconductive substance; then attaching a highly pulverized electroscopic material called toner, which is carried and transported by a developer carrying member, to the latent image to form a toner image corresponding to an electrostatic latent image; transferring the toner image to the surface of an image supporting member such as paper as required; and subsequently fixing the toner image with heat, pressure or solvent vapor to obtain a copy.
As a development method for visualizing an electric latent image with a toner, a powder cloud method, a cascade development method, a magnetic brush method, a method employing a conductive magnetic toner, etc. are known. In addition to the above methods, a so-called J/B development method comprising applying a bias electric field composed of an alternating current component and a direct current component to a space between a developer carrying member (development sleeve) and a photoconductive layer to carry out development is known. A typical example of the development method is a magnetic brush method. The magnetic brush method involves use of a two-component developer composed of a toner and a magnetic carrier. By using magnetic particles of steel, ferrite, etc. as a carrier, a developer comprising the magnetic carrier is retained on a developer carrying member having a magnet incorporated therein, and the developer is arranged in the shape of a brush on the developer carrying member by the action of the magnetic field of the magnet. When the magnetic brush is brought into contact with the electrostatic latent image surface on a photoconductive layer, only the toner in the developer is attracted from the brush to the electrostatic latent image, so that the electrostatic latent image is developed.
Carriers which constitute a two-component developer applied to the above development method are roughly classified into conductive carriers and insulating carriers. As a conductive carrier, an oxidized or non-oxidized iron powder is usually used. However, a developer having the iron powder carrier as a constituent triboelectrically charges a toner in an unstable manner. On the other hand, a typical insulating carrier is a resin-coated carrier comprising a carrier core material composed of a ferromagnetic material such as iron, nickel or ferrite having a surface uniformly coated with an insulating resin, generally. A developer with such an insulating resin-coated carrier very rarely causes adhesion of toner particles to the carrier surface as compared with the case of the conductive carrier as described above, exhibits easily controlled triboelectric charging between a toner and a carrier, and has excellent durability and a long useful life, advantageously, and is thus particularly suitable for a high-speed electronic copier.
Various properties are required for an insulating carrier. Particularly important properties include appropriate chargeability, impact resistance, abrasion resistance, good adhesion of a coating material to a core, and a uniform charge distribution. There has been proposed use of a resin with a small surface energy as a material for a coating layer to prevent spending of a carrier such as toner deposition, thereby improving durability of a developer. Specifically, it is said that a carrier coated with a silicone resin, a fluorine resin or the like is spent only with difficulty, and provides a developer with a long life. Japanese Patent Application Laid-Open No. S62-121462 discloses that a resin layer is improved by adding various silane coupling agents to a condensation reaction-type silicone resin.
Other problems in an electrophotographic carrier relate to endurance stability and environmental stability. Generally, problems with endurance stability include slow initial rising of the charge quantity, production of fog in the initial image, and a tendency toward an increased image density. Problems with endurance stability in the long-term copying include a reduced charge quantity, production of fog in the image and a tendency toward an increased image density. Generally, problems with environmental stability in a high humidity environment include a tendency toward a reduced charge quantity, production of fog in the image, an increase in the image density, and a tendency toward scattering of a toner. Problems with environmental stability in a low humidity environment include a tendency toward an increased charge quantity and a tendency toward a reduced image density.